Petroleum refiners produce desirable products such as turbine fuel, diesel fuel and other products known as middle distillates, as well as lower boiling hydrocarbonaceous liquids, such as naphtha and gasoline, by hydrocracking a hydrocarbon feedstock derived from crude oil or heavy fractions thereof. Feedstocks most often subjected to hydrocracking are gas oils and heavy gas oils recovered from crude oil by fractionation. A typical heavy gas oil comprises a substantial portion of hydrocarbon components boiling above about 371° C. (700° F.), usually at least about 50% by weight boiling above 371° C. (700° F.). A typical vacuum gas oil normally has a boiling point range between about 315° C. (600° F.) and about 565° C. (1050° F.).
Hydrocracking is generally accomplished by contacting in a hydrocracking reaction vessel or zone the gas oil or other feedstock to be treated with a suitable hydrocracking catalyst under conditions of elevated temperature and pressure in the presence of hydrogen to yield a product containing a distribution of hydrocarbon products desired by the refiner.
For the most efficient hydrocracking of a hydrocarbonaceous feedstock, the sulfur and nitrogen content must initially be low or significantly reduced prior to the hydrocracking step. The reduction of sulfur and nitrogen is conventionally conducted in a separate hydrotreating process or unit. Because of the existence of two separate reaction zones, i.e., hydrotreating and hydrocracking, the fresh hydrocarbon feed must be pumped up to the required operating pressure and heated to the required temperature for the hydrotreating catalyst. The effluent from the initial hydrotreating stage is depressured so that the product boiling range material can be separated using the appropriate fractionation facilities. The remaining unconverted hydrocarbon feedstock is then pumped again to the operating pressure of the hydrocracking stage and reheated according to the temperature requirements of the second stage hydrocracking catalyst for further conversion. This duplicate pressuring and heating of hydrocarbon feedstock represents energy inefficiency. In addition, additional equipment is required to maintain the separate reaction zones. If the hydrocarbon feed contains significantly large amounts of organic nitrogen, it is necessary to remove the ammonia produced by the first stage hydrotreating reaction zone before the resulting hydrocarbon passes over the hydrocracking catalyst, otherwise the ammonia formed would severely affect the cracking catalyst activity. It is desirable to accomplish the separation of the hydrogen sulfide and ammonia impurities from the hydrotreating reaction zone effluent without having to depressure, fractionate and repressure the streams to the hydrocracking reaction zone.
Although a wide variety of process flow schemes, operating conditions and catalysts have been used in commercial hydrocracking activities, there is always a demand for new hydroprocessing and hydrocracking methods which provide lower costs, more valuable product yields and improved operability.